In microwave bandpass filter design, transmission zeros (TZs) on one or both sides of the passband are frequently required in order to meet rejection requirements. Transmission zeros are often realized by couplings between non-adjacent resonators, often referred to as cross couplings.
Folded structures are often used to realize couplings between non-adjacent resonators. However, folded structures may not be suitable where there are structural constraints that require an inline configuration and/or input and output connectors on opposite sides of the two end resonators.
One technique used to realize transmission zeros for an inline configuration is to use a coupling probe embedded in the housing of the filter. Reference is now made to FIG. 1A in which an inline cross-coupled microwave bandpass filter 100 in accordance with the prior art is illustrated. The filter 100 includes a housing 102, six cavities 104a to 104f, six resonators 106a to 106f situated in the cavities 104a to 104f, an input port 108 extending into the first cavity 104a, and an output port 110 extending into to the sixth cavity 104f. The filter 100 also includes a coupling probe 112 extending into the first and third cavities 104a and 104c to realize coupling between the first and third resonators 106a and 106c. However, such a long coupling probe 112 generates unwanted resonances.
Reference is now made to FIG. 1B in which the frequency response of the bandpass filter 100 of FIG. 1A centered at 1.54 GHz is illustrated. It can be seen from FIG. 1B that in addition to generating a transmission zero 130 in the upper stop band, the coupling probe 112 resonates and generates a spike 132 in the lower stop band. Other disadvantages for such filters include the difficulty of tuning the cross-coupling.
Other techniques used to realize transmission zeros for an inline configuration include: (1) the extracted pole technique described in J. R. Rhodes and R. J. Cameron, “General extracted pole synthesis technique with application to low-loss TE011-mode filters,” IEEE Trans. Microwave Theory and Tech., vol. 28, pp. 1018-1028, September 1980; and (2) the application of non-resonating nodes described in S. Mari and G. Macchiarella, “Synthesis of inline filters with arbitrarily placed attenuation poles by using non-resonating nodes,” IEEE Trans. Microwave Theory and Tech., vol. 53, pp. 3075-3081, October 2005. However, both techniques require additional resonating or non-resonating structures.